Smoothing in sculpt mode but not by Smooth tool
So I guess the answer is in your question  ;)   it looks like if you use the Clay brush with a high Autosmooth you add geometry without deforming the general shape. Also, maybe he sometimes switches to the Smooth brush with the shift shortcut. Of course make sure that your Dyntopo is activated with a low enough Detail Size (the lower it will be the smaller the geometry will be) and the Subdivide Collapse and Relative Detail options activated (so that the detail size will depend on your zooming).